Phase Tracking in Communications Systems

ABSTRACT

The present invention includes a method of determining a phase estimate for an input signal having pilot symbols. The method includes receiving a plurality of pilot symbols, and then multiplying two or more pilot symbol slots by corresponding correlator coefficients to correct a phase estimate of the input signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/586,668, filed Oct. 26, 2006, which will issue as U.S. Pat. No.8,265,217 on Sep. 11, 2012, which claims benefit to U.S. ProvisionalApplication No. 60/730,376, filed Oct. 27, 2005, all of which areincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to phase tracking incommunication systems that use pilot symbols, including for example,satellite communication systems.

2. Background

For a communication system with a very low signal-to-noise ratio (SNR)such as satellite system, known pilot symbols are inserted periodicallyin the data frame for the purpose of phase tracking The pilot symbolsare used to correct for Doppler effects, as well as phase noise, thatcause phase tracking errors between the incoming carrier signal and alocal oscillator within the satellite receiver. Specifically, the pilotsymbols are used to estimate the phase of the incoming signal (hereincalled the phase estimate). For example, the phase estimate can be anestimate of the phase delay introduced in the carrier signal by theDoppler effects.

For example, FIG. 1 illustrates a data steam with pilot symbol slots 102a-d that are inserted in the data stream where each symbol slot can haveone or more pilot symbols. The pilot symbols are used to generate thephase estimates, and the phase estimates are used to align the phase ofthe incoming signal with the phase of the local oscillator in thedigital receiver portion of a satellite receiver. This alignment processis known as phase tracking or phase correction. In order to minimize theoverhead for pilot symbols, the number of pilot symbols in each slot iskept as small as possible.

Conventional communication systems use phase estimates that are based onone slot of pilot symbols to adjust the phase of the receiver's localoscillator. However, a problem can occur in low SNR communicationsystems. In these low SNR systems, phase estimates that are based on oneslot of pilot symbols may not be sufficient to accommodate the requiredsystem performance.

What is needed, therefore, is an apparatus and method of determining thephase estimate of the incoming data signal using multiple pilot symbolslots and then using those phase estimates for the purpose of phasetracking

BRIEF SUMMARY OF THE INVENTION

The present invention includes a method of determining a phase estimatefor an input signal having pilot symbols. The method includes receivinga plurality of pilot symbols, and then multiplying two or more adjacentpilot symbol slots by corresponding correlator coefficients to produce aplurality of correlator outputs that are combined to produce phaseestimate of the input signal.

The present invention provides a technique for providing phaseestimations that use pilot symbols from multiple slots, spaced apart toachieve a better estimate than those of prior art systems that use onlyone slot. More specifically, the present invention considers one or moreadjacent (e.g., neighboring) pilot symbol slots to perform the phaseestimation that is used for phase tracking. Traditional systems, asnoted above, typically use only one slot of pilot symbols to perform thephase estimation that is used for the phase tracking. By considering oneor more neighboring pilot symbol slots, the present invention decreasesthe phase estimation variance in communication systems where the SNR isseverely compromised, thereby enhancing the system's phase trackingcapabilities.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left-most digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

FIG. 1 is an illustration of a data stream having pilot symbols embeddedtherein.

FIG. 2 is a block diagram illustration of a phase estimation andtracking system that receives incoming data, phase corrects the incomingdata, and produces output data in accordance with the present invention.

FIG. 3 is a block diagram illustration of a conventional phase estimatorbased on the determination of a phase estimate using one pilot symbolslot.

FIG. 4 is a block diagram illustration of a phase estimator that weightsphase estimates from multiple pilot symbol slots over time according toembodiments of the invention.

FIG. 5 is a flow chart of an exemplary method of practicing anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the present invention refers tothe accompanying drawings that illustrate exemplary embodimentsconsistent with this invention. Other embodiments are possible, andmodifications may be made to the embodiments within the spirit and scopeof the invention. Therefore, the following detailed description is notmeant to limit the invention. Rather, the scope of the invention isdefined by the appending claims.

It will be apparent to one skilled in the art that the presentinvention, as described below, may be implemented in many differentembodiments. Any actual software code implementing the present inventionis not limiting of the present invention. Thus, the operational behaviorof the present invention will be described with the understanding thatmodifications and variations of the embodiments are possible, given thelevel of detail presented herein.

FIG. 1 is a block diagram illustration of a data stream 100 in the timedomain having embedded pilot symbol slots 102(a-d) to provide for phaseestimation and correction of phase noise. Each slot consists of one ormore known symbols, also known as pilot symbols. Segments (d) within thedata stream 100 are representative of payload data. In the exemplaryembodiment of FIG. 1, a spacing between pilot symbol slots, for example,between the symbols 102(a) and 102(b) is 1440 symbols. However, othersystems could use other spacing values that would also be within thespirit and scope of the present invention.

FIG. 2 is a block diagram illustration of an architecture of a phaseestimation and correction system 200 that uses pilot symbols to providephase estimates. Stated another way, the tracking scheme of the system200 is used to align the phase of the incoming data with the localoscillator phase in the satellite receiver. The system 200 receives anincoming input data stream 202, phase corrects the incoming data, andproduces output data 218. The system 200 includes a delay buffer 204, amultiplier 206, a phase estimate module 208, a summer 210, a processingmodule 212, a delay 216, and a pilot flag module 213 a.

In FIG. 2, the pilot flag module 213 a produces an enable phase estimatesignal 213 b to enable the phase estimate module 208. The pilot enablesignal 213 b is generated when the pilot flag module 213 a senses thepresence of a pilot symbol slot within the input data stream 202, suchas any one of the pilot symbol slots 102(a-d), shown in FIG. 1. That is,the phase estimate module 208 is only activated to estimate phase whenthe module 208 is triggered by the enable 213 b. The phase estimatemodule 208 generates a current phase estimate 214 that is used to alignthe input data stream 202 with the phase of the local oscillator.

During operation of the system 200, the input data stream 202 is storedin the delay buffer 204. The pilot symbols within the slots 102(a-d) arestripped from the input data stream 202 and forwarded as a pilot symbolsignal 203 to the phase estimate module 208, to produce the currentphase estimate 214 as an output.

After the phase of the pilot symbol signal 203 is estimated within thephase estimate module 208, the current phase estimate value 214 isoutput from the phase estimate module 208. For example, the phase can beestimated for the pilot symbol slot 102(b) of the data stream 100. Aprevious phase estimate value (e.g., previous phase estimate for thepilot symbol slot 102(a)), is stored within the delay module 216. Theprevious phase estimate value for 102(a) is then compared with thecurrent phase estimate value 214 (representative of the phase of 102(b))within the summer 210 to produce a difference phase estimate value 211.

The difference phase estimate value 211, which represents a phasedifference between the pilot symbol slots 102(a) and 102(b), is latershaped as a linear phase ramp. This ramp is then converted to a complexsinusoidal value that is applied to the data stored within the delaybuffer 204. This aspect of the present invention is discussed in greaterdetail below.

The processing module 212 is configured to produce a complex sinuoisdalsignal that is based on a linear phase ramp. The complex sinusoidalsignal is applied to the data output from the delay buffer 204. Theprocessing module 212 includes a scalar 220 and a digital directfrequency synthesizer (DDFS) module 222. The scalar 220 divides thedifference phase estimate value 211 by the number of data symbols (e.g.,1440 symbols), between the pilot symbols 102(a) and 102(b). The DDFS 222then produces the linear phase ramp based on the divided differencephase estimate. The ramp start and end points correspond to the phaseestimate values for pilot symbol slots 102(a) and 102(b), respectivelyThe linear phase ramp is produced based upon an output from the scalar220 in accordance with techniques well known to those of skill in theart.

The linear phase ramp is produced by the DDFS 222, which then convertsthe phase ramp to a complex sinusoidal value. More specifically, thereal part of the DDFS output corresponds to the sine of the phase ramp,and the imaginary part of the DDFS output corresponds to the cosine ofthe phase ramp. The output from the DDFS 222 is then used to correct thephase of the input data stream 202 that is stored within the delaybuffer 204. More specifically, the DDFS outputs are multiplied with theoutput from the delay buffer 204 to produce phase corrected output data218. In this manner, a phase correction is applied to data from the datastream 100 that lies between the pilot symbol slots 102(a) and 102(b),using phase estimates based on the pilot symbol slots 102(a) and 102(b).

FIG. 3 illustrates a conventional phase estimator 300 configured toproduce a phase estimate 314. The conventional phase estimator 300includes a data register 302, multipliers 304 a-c, a pilot symbolcorrelator coefficient generator 305, and a complex number to phaseconversion unit 308.

During operation, the pilot symbols 203 are shifted into the dataregister 302. In the phase estimator 300, the number of multiplierswithin the data register 302 corresponds to the number of pilot symbolswithin the data stream 100. The multipliers 304 multiply correspondingpilot data by pilot symbol coefficients that are generated by thecorrelator coefficient generator 305, to retrieve the actual pilotsymbol data. By way of example, the coefficients can be conjugates ofthe corresponding pilot symbols.

The resulting outputs from the multipliers 304 a-c are added togetherusing the adder 306 to produce a correlator output signal 307. Thecorrelator output signal 307 is processed by the complex number to phaseconversion unit 308 to convert the correlator output signal 307 to aphase, which is the phase estimate 314.

The phase estimate is calculated using a coherent correlator followed bya complex number to phase conversion unit, as shown in FIG. 3. Given asequence of pilot symbols P, the correlator coefficient vector C can beexpressed as follows:

P={e^(jφ) ⁰ , e^(jφ) ¹ , e^(jφ) ² , e^(jφ) ³ , . . . }

C={e^(−jφ) ⁰ , e^(−jφ) ¹ , e^(−jφ) ² , e^(−φ) ³ , . . . }

The phase estimate variance is limited by the number of symbols in apilot slot. Since the estimate variance determines the overall receiverperformance, it is desirable to make the phase estimate variancesmaller. A smaller phase estimate variance can be achieved using longerpilot slots, but that costs system bandwidth. On the other hand, thephase change in a receiver is gradual when the system is locked. Usingthis property, a weighted function can be used to smooth out the currentphase estimate. An implementation of this weighted phase estimate isshown in FIG. 4. In the implementation of FIG. 4, an earlier phaseestimate and a later phase estimate are used. This architecture canreduce the estimation variance by a factor of three.

In general, the variance of phase estimates in pilot symbol basedcommunication systems, such as the estimator 300, is dependent on thenumber of symbols used to generate the estimate. The phase estimate 314is only based on one pilot symbol slot, using 36 symbols. Since thesystem 300, which does not have the ability to change the number ofsymbols in a pilot symbol slot, uses only one pilot symbol slot tocreate its estimate, it may not be robust enough in a high noiseenvironment. A greater number of symbols (e.g., 72, 108, of higher)would provide an estimate with lower variance, which would ultimatelytranslate to output payload data with a high signal to noise ratio.

FIG. 4 is a block diagram illustration of a phase estimator 400constructed in accordance with an embodiment of the present invention.The phase estimator 400 of FIG. 4 weighs two or more phase estimatesover time in order to improve the phase estimate in a noisy environment.Based upon simulations and test measurements illustrating that a greaternumbers of pilot symbols reduces the phase estimation variance, thephase estimator 400 is configured to change the number of symbols usedin the estimate. So, for example, instead of being restricted to usingonly the current pilot symbol slot to create the estimate, neighboringpilot symbol slots can also be used.

Phase estimator 400 includes the data register 302, the multipliers 304,the coefficient generator 305, and the adder 306. However, the phaseestimator 400 also includes a weight coefficient generator 402, delays404 a and 404 b, multipliers 406 a-c, the adder 408, and a complexnumber to phase converter 401. Note that there could be more than twodelays 404 depending the number of phase estimates that are averaged,which is determined by the specific embodiment.

In addition to the functions discussed above for the phase estimator300, the phase estimator 400 also weights and averages one or more phaseestimates over time (past, present, and future). The correlator output307 is delayed by first and second delays 404 a and 404 b. Accordingly,the multipliers 406 a-c process a past, a present, and a futurecorrelator output 307. By doing so, the phase estimator 400 averagespast, present, and future phase estimates to produce a final phaseestimate 414, where the average is performed in a weighted fashion.

Specifically, the multiplier 406 a multiplies the correlator output 307by a first weighting coefficient, to produce a first weighted correlatoroutput 407 a. The weight coefficient generator 402 produces a set ofweighting coefficients based upon user calculations. That is, thecoefficients are pre-calculated based upon characteristics of thecommunication system.

The multiplier 406 b multiplies the correlator output 307 b by a secondweighting coefficient, to produce a second correlator output 407 b. Themultiplier 406 c multiplies the correlator output 307 c by a thirdweighting coefficient, to produce a third correlator output 407 c. Theweighted correlator outputs 407 a-c are summed by the adder 408, toproduce a combined correlator output 411. The complex number to phaseconverter 401 converts the combined correlator output 411 to a phase,which provides the weighted phase estimate output 414 that is used forthe phase estimate 214 in FIG. 2.

Note that any number of correlator outputs 307 can be weighted andaveraged. Thus, the present invention is not limited to three as shown.

Regarding the weighting coefficients, in a slow moving channel, an equalweighting can be used: ⅓ (past), ⅓ (present), ⅓ (future). In oneembodiment, a weighting of ¼ (past), ½ (present), ¼ (future).

FIG. 5 is a flow chart of an exemplary method 500 of practicing anembodiment of the present invention. In the method 500, a plurality ofpilot symbol slots is received in a step 502. In step 504, two or moreadjacent pilot symbol slots are multiplied by corresponding correlatorcoefficients to produce a plurality of correlator outputs to correct aphase estimate of the input signal. The plurality of correlator outputsare averaged. The result of the averaging is then converted from acomplex representation to a phase representation.

CONCLUSION

Example embodiments of the methods, systems, and components of thepresent invention have been described herein. As noted elsewhere, theseexample embodiments have been described for illustrative purposes only,and are not limiting. Other embodiments are possible and are covered bythe invention. Such other embodiments will be apparent to personsskilled in the relevant art(s) based on the teachings contained herein.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A system for aligning a phase of data symbols positioned between atleast two pilot symbol slots in a received data stream, one of the slotsbeing currently received and the other slot being previously received,the system comprising: a flag module configured to produce an enablesignal when at least one of the pilot symbol slots is sensed; anestimator for (i) estimating a phase of a first pilot symbol in thecurrently received slot and (ii) producing a current phase estimatevalue based upon the enable signal; and a comparator for comparing thecurrent phase estimate value with a phase estimate value from a secondpilot symbol of the previously received slot to produce a differencephase estimate value; wherein a phase of the data symbols is correctedbased upon the difference phase estimate value.
 2. The system of claim1, further comprising a delay module for delaying the phase estimatevalue of the first pilot symbol signal.
 3. The system of claim 2,wherein the delay module includes a memory for storing the delayed phaseestimate value.
 4. The system of claim 3, wherein the difference phaseestimate value is representative of a difference between the at leasttwo pilot symbol slots.
 5. The system of claim 4, further comprising aprocessing module for producing a phase ramp representative ofrespective beginning and ending points of the first and second pilotsymbols.
 6. The system of claim 5, wherein the processing moduleincludes a scalar and a frequency synthesizer.
 7. The system of claim 6,wherein the phase ramp is converted to a complex sinusoidal value. 8.The system of claim 7, further comprising a multiplier coupled to thedelay module and the processing module.
 9. The system of claim 8,wherein the multiplier is configured for multiplying the complexsinusoidal value with the data symbols.
 10. The system of claim 9,wherein the multiplying corrects the phase of the data symbols.
 11. Atangible computer readable medium having stored thereon computerexecutable instructions that, if executed by a computing device, causethe computing device to perform a method for aligning a phase of datasymbols positioned between at least two pilot symbol slots in a receiveddata stream, one of the slots being currently received and the otherslot being previously received, the method comprising: generating anenable signal when at least one of the pilot symbol slots is sensed;estimating via a phase estimator a phase of a first pilot symbol in thecurrently received slot to produce a current phase estimate value basedupon the enable signal; comparing the current phase estimate value witha phase estimate value from a second pilot symbol of the previouslyreceived slot to produce a difference phase estimate value; andcorrecting a phase of the data symbols based upon the difference phaseestimate value.
 12. The tangible computer readable medium of claim 11,wherein the comparing includes storing and delaying the phase estimatevalue of the first pilot symbol signal.
 13. The tangible computerreadable medium of claim 12, wherein the difference phase estimate valueis representative of a difference between the at least two pilot symbolslots.
 14. The tangible computer readable medium of claim 13, whereinthe correcting includes producing a phase ramp representative ofrespective beginning and ending points of the first and second pilotsymbols.
 15. The tangible computer readable medium of claim 14, whereinthe phase ramp is converted to a complex sinusoidal value.
 16. Thetangible computer readable medium of claim 15, wherein the correctingincludes multiplying the complex sinusoidal value with the data symbols.